An auto-ignition internal combustion engine may be used as a motor vehicle drive unit. Within the context of the present disclosure, the expression “internal combustion engine” encompasses diesel engines, which utilize a combustion process with auto-ignition, and hybrid drives comprising the auto-ignition internal combustion engine and also an electric machine that can be connected in terms of drive to the internal combustion engine and receives power from the internal combustion engine or, as a switchable auxiliary drive, additionally outputs power.
On account of the limited availability of mineral oil as raw material for the production of fuels and the increase of global warming as a result of the greenhouse effect, it is constantly sought in the development of internal combustion engines to minimize fuel consumption. Furthermore, a reduction of the pollutant emissions is fundamentally sought in order to comply with future limit values for pollutant emissions. Therefore, the development of fuel-consumption-optimized combustion methods is at the forefront of internal combustion engine development efforts.
A problem is fuel consumption owing to the relatively poor efficiency in particular of Otto-cycle engines. The reason for this lies in the principle of the operating process of the traditional Otto-cycle engine. The conventional diesel-engine method is afflicted, in particular, with high nitrogen oxide emissions due to high temperatures resulting from the combustion process and with high soot emissions owing to the inhomogeneous fuel-air mixture.
For reducing the emissions of an internal combustion engine, a distinction can be made between two fundamentally different approaches.
A first approach for reducing the emissions includes aftertreatment of the exhaust gas that is formed during combustion, and of the pollutants contained therein. To reduce pollutant emissions, internal combustion engines may be equipped with various exhaust gas aftertreatment systems.
In a second approach, it is sought to influence the combustion process such that the fewest possible pollutants arise (e.g., are formed) during the combustion of the fuel in the first place. Since, for example, the formation of nitrogen oxides takes place during an excess of air and high temperatures, combustion methods with relatively low combustion temperatures are expedient for reducing nitrogen oxide emissions. Low combustion temperatures may be realized, for example, by increasing ignition retardation and/or reducing the rate of combustion. Both can be achieved by admixing combustion gases to a cylinder fresh charge and/or by increasing the exhaust gas fraction in the cylinder fresh charge. As a result, exhaust gas recirculation (EGR) is to be regarded as a suitable measure for lowering the combustion temperature. With increasing EGR rate, the nitrogen oxide emissions can be considerably reduced. Here, the nitrogen oxide emissions and also the soot emissions decrease.
For the stated reasons, an increasing number of new combustion methods are being developed and tested. An example of a combustion method of said type is the homogeneous charge compression ignition (HCCI) method, which is also referred to as the spatial ignition method or controlled auto-ignition (CAI) method and which is based on a controlled auto-ignition of the fuel supplied to a cylinder. Here, the fuel is generally burned with an excess of air (e.g., superstoichiometrically). Owing to the low combustion temperatures, an internal combustion engine operated in the HCCI mode exhibits relatively low nitrogen oxide emissions and likewise low, or virtually absent, soot emissions. Additionally, owing to the relatively low combustion temperatures and the associated relatively low temperature differences in the internal combustion engine, heat losses are lower than in the case of conventionally operated internal combustion engines. This leads to a higher thermal efficiency.
An HCCI method and an internal combustion engine that uses said method for the combustion of fuel are described in U.S. Pat. No. 6,390,054 B1, wherein the internal combustion engine that is used is an applied-ignition Otto-cycle engine.
However, the inventors herein have identified potential issues with such systems. As one example, the HCCI method cannot be used at all operating points of the internal combustion engine, such that the advantages described above can be utilized in a small region of the engine characteristic map (load versus engine speed). For example, the HCCI method can be used to a limited extent at relatively high loads and relatively high engine speeds. With increasing load, owing to the decreasing air ratio, the auto-ignition is shifted in an advancing direction, (e.g., the fuel-air mixture ignites at an earlier time in the compression phase), wherein the conversion rates and/or the rate of combustion likewise increase. Owing to the early ignition time, which is inconsistent, and in particular owing to the relatively fast rate of combustion, the running of the internal combustion engine becomes more irregular and rough. Furthermore, owing to the heat of combustion that is released long before the top dead center, the thermal efficiency is reduced. With increasing engine speed, the time that is available for the preparation of the fuel-air mixture, in particular for the homogenization, is shortened, such that, at high engine speeds, it is not possible for an adequately homogenized fuel-air mixture to be generated. Furthermore—assuming a successful auto-ignition—the focus of the combustion is shifted in a retarding direction, such that the thermal efficiency of the combustion process is impaired. This is also the reason why the internal combustion engine cannot be operated exclusively using the HCCI method, as a further combustion method is utilized so that the internal combustion engine can be operated at the operating points at which the HCCI method fails.
To make the HCCI method suitable for broad characteristic map regions, and to expand the range of use of said method, various concepts have been developed. In particular, it is sought, by various measures, to influence the temperature of the fresh charge supplied to the cylinder, for example, by way of an internal and/or external recirculation of exhaust gas. Despite these efforts, it has still not been possible to operate an internal combustion engine exclusively (e.g., at all operating points of the engine characteristic map) in accordance with the HCCI method, such that the use of other combustion methods aside from the HCCI method is unavoidable. Furthermore, if the temperature of the cylinder fresh charge, the control of the ignition time, and/or the control of the combustion profile are/is influenced by way of exhaust gas recirculation (EGR), a problem is posed in particular by the dynamic control of the EGR (e.g., the recirculation rate during transient operation of the internal combustion engine), wherein adequate dynamics (e.g., a satisfactory response behavior of the EGR) cannot be realized in a satisfactory manner. Further still, the HCCI method leads to relatively high emissions of carbon monoxide and unburned hydrocarbons.
The above-described problem is intensified in the case of internal combustion engines with exhaust gas turbocharging. If exhaust gas is extracted from the exhaust gas discharge system by way of high-pressure EGR upstream of the turbine, the recirculated exhaust gas is no longer available for driving the turbine, whereby the turbocharging is adversely affected. In the event of an increase in the EGR rate, the exhaust gas flow introduced into the turbine simultaneously decreases. The reduced exhaust gas mass flow through the turbine leads to a lower turbine pressure ratio. As a result, the charge pressure ratio also falls, which equates to a smaller compressor mass flow. Aside from the decreasing charge pressure, additional problems may arise in the operation of the compressor with regard to the surge limit of the compressor. Disadvantages may also arise in terms of the pollutant emissions, for example, with regard to the formation of soot during an acceleration in the case of diesel engines.
In the case of a low-pressure EGR arrangement, it is duly the case that exhaust gas that has already flowed through the turbine is recirculated into the intake system. However, owing to the relatively long paths to reach cylinders of the engine and larger volumes, the low-pressure EGR arrangement is more inert to transient changes. The low-pressure EGR arrangement also has a poorer response behavior, because, in the context of low-pressure EGR, exhaust gas is used which has undergone exhaust gas aftertreatment, in particular by a particle filter, downstream of the turbine. In this way, depositions in the compressor which change the geometry of the compressor, in particular the flow cross sections, and impair the efficiency of the compressor can be prevented. Further, the driving pressure gradient between the exhaust gas discharge system and the intake system is generally smaller in the case of a low-pressure EGR arrangement, such that it is not possible to realize arbitrarily high recirculation rates.
In one example, the issues described above may be at least partly addressed by a system for an auto-ignition internal combustion engine, comprising an exhaust gas recirculation arrangement; an intake system for supplying charge air to the auto-ignition internal combustion engine; an exhaust gas discharge system for the discharge of exhaust gas; and a three-way catalytic converter in the exhaust gas discharge system configured to reduce nitrogen oxides and oxidize unburned hydrocarbons and carbon monoxide in the exhaust gas when the internal combustion engine is operated in a stoichiometric first operating mode, in which the stoichiometric first operating mode is a homogenous charge compression ignition mode. In this way, an auto-ignition internal combustion engine may be operated in the HCCI mode with reduced pollutant emissions.
As one example, the system may further comprise an exhaust gas recirculation system, a bypass passage for bypassing a charge-air cooler, and a bypass passage for bypassing an intercooler. The bypass passages may be opened to reduce intake volume, thereby increasing EGR dynamics, and increase the temperature of the fresh charge. Thus, the system described herein may provide sufficient EGR dynamics and temperature control to control auto-ignition timing in the HCCI mode and increase the operating range of the HCCI mode.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.